A novel enantiopure tricyclic beta-lactam via stereoselective intramolecular nitrilimine cycloaddition

Starting from the Staudinger [2+2] cycloaddition between the 1-azadiene 1 and acetoxyacetylketene, generated in situ from acetoxyacetyl chloride, we developed a seven-step synthesis of the novel azeto[3′,4′:2,3]pyrano[4,5-c]pyrazole skeleton 9 in both racemic and enantiopure forms. Silver carbonate treatment of the functionalised hydrazonoyl chloride 7 promoted in situ generation of the corresponding nitrilimine 8, which underwent a clean, stereoselective cycloaddition to the target tricyclic β-lactam 9. The key step of the synthetic pathway leading to enantiopure 9 was the production of the enantiopure azetidinone (3R,4S)-3 by enzymatic resolution with Amano Lypase PS of the racemic precursor (3S*,4R*)-2.